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Australian researchers have found that long-term heavy use of marijuana may cause parts of the brain to shrink.

Published in this month's Archives of General Psychiatry, the study found that the hippocampus and amygdala were smaller in men who were heavy marijuana users compared to non-users.

The study looked at 15 men heavy marijuana users, who had smoked at least five marijuana cigarettes daily for on average of 20 years.

Brain scans showed that on average their hippocampus volume was 12% less and amygdala volume was 7% less than in the 16 men who were not marijuana users.

The hippocampus regulates memory and emotion, while the amygdala plays a critical role in fear and aggression.

"These findings challenge the widespread perception of cannabis as having limited or no harmful effects on (the) brain and behaviour," says Dr Murat Yucel of ORYGEN Research Centre and the University of Melbourne, who led the study.

The researchers also found heavy cannabis users earned lower scores than non-users in a verbal learning task - trying to recall a list of 15 words.

Psychotic disorders

The users were more likely to exhibit mild signs of psychotic disorders, but not enough to be formally diagnosed with any such disorder, the researchers say.

"Like with most things, some people will experience greater problems associated with cannabis use than others," Yucel says.

"Our findings suggest that everyone is vulnerable to potential changes in the brain, some memory problems and psychiatric symptoms if they use heavily enough and for long enough."

While about half of the marijuana users reported experiencing some form of paranoia and social withdrawal, only one of the non-users reported such symptoms, Yucel says.

The heavy marijuana users, average age 40, said they had used other illicit drugs less than 10 times.

The researchers acknowledged that the study's findings could not be completely attributed to marijuana use.

But Yucel says the findings certainly suggested marijuana was the cause.

He says the researchers have begun new research on the effects of short-term, long-term, moderate and heavy use of marijuana.